It is now a widely accepted fact that marine heatwaves (MHWs) are caused by either anomalous heat fluxes into the ocean or by anomalous advection across steep thermal gradients. A literature review has been performed that investigated the known driver of many of the recorded MHWs (Holbrook et al., 2019). In this same paper the researchers also showed which climate indexes (e.g. PDO) most closely related to the annual occurrence of MHWs with good success. There is however still work to be done on understanding the drivers of MHWs at the local/sub-mesoscale. In this talk I will outline how my coauthors and I have decided to investigate, quantify, and conclude on the potential drivers of MHWs in the North West (NW) Atlantic.
NW Atlantic with regions from Richaud et al. (2016).
Surface heat flux (W/m2):
Mixed layer depth:
Surface movement:
Surface temperature:
(Benthuysen et al., 2014; Chen et al., 2015; Oliver et al., 2017)
This statistic allows us to determine how similar the daily change in SSTa during a MHW is with the daily change in any Qx term. In order to do so we must first ensure that the daily Qx term is an integral of the SSTa. This was accomplished by first creating a 12 hour lag with the daily SSTa after the daily Qx terms. THis was possible because the ERA5 data are hourly, not daily data. With the integral created, the daily Qx terms were then divided by the density & specific heat of sea water as well as MLD:
Qx/ρ0cpH
where Qx is the given heat flux variable, ρ0 = 1042, cp ~= 4000, and H is the MLD on a given day. This effectively converted the Qx terms into units of °C/day. The final step in the data prep before running the RMSE was to create cumulative Qx terms during a MHW. This was done by taking the Qx on the first day of an event, and cumulatively adding the following days until the end of the event. These cumulative Qx terms were then added to the SSTa from the first day of the MHW to show how much of the daily change could be attributed to a given Qx term. For example:
SSTa1 -> SSTa2 -> SSTa3 -> SSTa4
SSTa1 -> SST1a+Qx1 -> SST1a+Qx2 -> SST1a+Qx3
Because the other variables used in this study are not in units of °C/day we cannot directly compare them to SSTa with the RMSE statistic. Instead we must use a correlation. This is a very simple test but it does not show causality and it is also weak to giving high or low values when the scale of change between variables may not be the same. For example, SSTa may show very large fluctuations, and precipitation may show very small (possibly meaningless) fluctuations. But as long as the fluctuations in each variable are relatively large compared to their central tendency this will produce a strong correlation.
The mean air-sea states during each MHW were fed to a SOM to produce a 12 node (4x3) grid of the most representative synoptic patterns that may be forcing MHWs. This was done to provide a broader view of what may be driving MHWs. The following figure is a schematic showing what the data for a single MHW given to the SOM looks like. There were 291 total MHWs.
Let’s head over to the shiny app for an interactive tour of the results: https://robert-schlegel.shinyapps.io/MHWflux/
Benthuysen, J., Feng, M., and Zhong, L. (2014). Spatial patterns of warming off Western Australia during the 2011 Ningaloo Niño: quantifying impacts of remote and local forcing. Continental Shelf Research 91, 232–246. doi:10.1016/j.csr.2014.09.014.
Chen, K., Gawarkiewicz, G., Kwon, Y.-O., and Zhang, W. G. (2015). The role of atmospheric forcing versus ocean advection during the extreme warming of the Northeast U.S. continental shelf in 2012. Journal of Geophysical Research: Oceans 120, 4324–4339. doi:10.1002/2014JC010547.
Holbrook, N. J., Scannell, H. A., Gupta, A. S., Benthuysen, J. A., Feng, M., Oliver, E. C., et al. (2019). A global assessment of marine heatwaves and their drivers. Nature communications 10, 1–13.
Oliver, E. C. J., Benthuysen, J. A., Bindoff, N. L., Hobday, A. J., Holbrook, N. J., Mundy, C. N., et al. (2017). The unprecedented 2015/16 Tasman Sea marine heatwave. Nature Communications 8, 16101. doi:10.1038/ncomms16101.
Richaud, B., Kwon, Y.-O., Joyce, T. M., Fratantoni, P. S., and Lentz, S. J. (2016). Surface and bottom temperature and salinity climatology along the continental shelf off the canadian and us east coasts. Continental Shelf Research 124, 165–181.